1. Field of the Invention
This invention relates to a method of manufacturing an optical element. More particularly, the present invention relates to a method for manufacturing a composite optical element.
2. Description of the Related Art
A composite optical element can constitute a transparent plastic material molded on a transparent optical element (such as glass or quartz). In general, from the optical point of view, aberrations exist on spherical lenses. If the surface of a lens is manufactured into a non-spherical shape, the formation of the spherical aberration will be reduced.
From the viewpoint of manufacturing process, the processing time and cost for non-spherical glass lenses are much higher than those for spherical glass lenses. Nevertheless, as molding technique further develops, plastic materials are more and more often being employed in the production of optical lenses. Workability of plastic is much better than that of glass, and it is cheaper to form a non-spherical lens from plastic than from glass. Moreover, plastic is lighter, has better tintability and shock resistance, and has a lower processing temperature than that of glass. But glass has better radiation resistance, durability and scratch resistance, and surface treatment is easier for glass than for plastic. Therefore, a composite lens that combines both glass and plastic materials will possess characteristics of both.
However, such a composite lens has two main problems. One is the big difference between the coefficients of thermal expansion of plastic and glass. The second one is that the binding between plastic and glass is not good enough. In general, delamination or cracking of the composite lens can occur under variation of temperature if the difference in the coefficients of thermal expansion of materials is too large. Moreover, molecular structures and functional groups cannot provide effective chemical bonds or physical entanglements between plastic and glass. Hence, the interface between glass and plastic lacks a strong binding.
In order to cope with the weaknesses described above, a layer of adhesive or coupling agent is coated between the plastic layer and the glass layer in the prior art such as the one disclosed in the U.S. Pat. No. 4,793,703. Alternatively, a coupling agent is added into the plastic layer such as the one disclosed in the Japanese Patent JP07306305. A holding force due to the encapsulation of the glass in the plastic may even be employed, such as those disclosed in the Japanese Patents JP59046601, JP01221701 and JP08190004. Addition of an adhesive or a coupling agent eventually increases the costs of material and equipment because an additional material is used. The environmental effect needs to be considered as well. The efficiency of composite lens production is low due to the many steps in the manufacturing process. Furthermore, the encapsulation method, which requires a plastic layer be extended from an optical surface of a glass lens over a lateral edge to the periphery of another optical surface, makes the structure of the molding apparatus more complicated.